The present invention relates to a magnetic recording medium for a hard disk drive and a method of manufacturing the medium, and more particularly, to a medium having a higher signal to noise ratio and lower manufacturing cost.
With the development of modern information processing techniques, high recording density is required of a magnetic disk apparatus to be used as an external memory device of a computer. In more practical terms, it is now a trend to use, within a reproducing head of the magnetic disk apparatus, a magneto-resistive head, namely, an MR head utilizing a magneto-resistive element in place of the coil type inductive thin film magnetic head of the prior art. The MR head utilizes the magneto-resistive effect, in which electrical resistance of a magnetic material changes depending on the external magnetic field, for reproduction of a signal on the recording medium. The MR head has several advantages over the inductive thin layer magnetic head of the prior art. Among other things, inductance is smaller and a large signal to noise ration (S/N) can be expected. Moreover, an AMR head utilizing the anisotropic magneto-resistive effect, a GMR head utilizing a gigantic magneto-resistive effect and a spin valve GMR head are also used in addition to such MR heads.
Moreover, in order to satisfy the requirement of high density recording, the magnetic recording medium to be used in the magnetic disk apparatus is also required to have performance characteristics needed for the MR head, AMR head or GMR head (including the spin valve head). The magnetic recording medium is required to have higher coercive force Hc to improve the S/N in view of the high recording density.
In the magnetic recording medium of the prior art, a magnetic layer consisting of an alloy including cobalt is formed on a non-magnetic substrate such as an aluminum substrate. Moreover, a non-magnetic underlayer consisting of Cr or its alloy is provided between the substrate and magnetic layer. This non-magnetic underlayer is intended to set the easy magnetizing direction of the magnetic layer within the layer surface. Particularly, in order to attain the intensity, in the magnetic recording medium using aluminum as the substrate, an NiP layer is formed by the plating and sputtering method on the surface of the substrate.
In order to reduce noise in such a magnetic recording medium, an alloy element is added to reduce the exchange interaction between magnetic particles of the magnetic layer, and the grain size of the magnetic particles forming the magnetic layer is reduced. For example, the Published Unexamined Japanese Patent Application No. S63-148411 discloses a magnetic recording medium which is suitable for high density recording apparatus. In that magnetic recording medium, the magnetic layer includes any one of Ta, Mo, W or an alloy thereof as a third additional element to the Coxe2x80x94Ni based alloy or Coxe2x80x94Cr based alloy.
Another method of reducing noise and obtaining excellent S/N is to perform a texture process in the circumferential direction of an aluminum substrate having a non-magnetic NiP layer at the surface thereof. The direction for easy magnetization of the magnetic layer is further directed to the circumferential direction by performing the texture process of the substrate surface in the circumferential direction. As a result, improvement of S/N can be realized.
In a magnetic disk apparatus to be loaded in a portable apparatus such as a notebook size personal computer or the like, it is desirable to use a non-magnetic substrate such as glass or the like having excellent impact-proof characteristics as the substrate of a magnetic recording medium in place of the aluminum substrate, which is weaker in strength. Many examples of magnetic recording media having glass substrates are reported.
For example, the Published Unexamined Japanese Patent Application No. H7-73427 discloses a magnetic recording medium 20 comprising, as the cross-sectional view is illustrated in FIG. 1, a basic layer including a glass substrate 21, a first layer (including chromium) 22 formed in the non-magnetic substrate side of the glass substrate 1, a second layer (including chromium and molybdenum) 23 formed in the magnetic layer side, and a magnetic layer 24 including cobalt and platinum. On the magnetic layer 24, a protective layer 25 consisting of a chromium layer 25a and a carbon layer 25b is also provided, and a lubricant layer 26 is provided on the protective layer 25.
Moreover, the Published Unexamined Japanese Patent Application No. H8-227516 discloses a magnetic recording medium 30 comprising, as the cross-sectional view is illustrated in FIG. 2, a glass substrate 31 and an aluminum (Al) thin layer 32a, a Cr thin layer 32b, a CoMo thin layer 32c, a first magnetic layer 33, a non-magnetic layer 34, a second magnetic layer 35, a first protective layer 36a, a second protective layer 36b and a lubricating layer 37 which are sequentially formed on such a glass substrate.
However, in the magnetic recording medium using a glass substrate, a satisfactory S/N cannot be obtained only by forming directly on the substrate a Cr-based underlayer such as Cr, CrMo. Actually, when a glass substrate is used in the magnetic recording medium, it has been confirmed that medium noise increases abnormally in the ordinary Cr-based underlayer.
Therefore, as another method of obtaining good S/N of the magnetic recording medium having the glass substrate, it is considered to form the NiP layer as the seed layer on the glass substrate by the plating method or sputtering method, and then perform the texture process in the circumferential direction of the NiP layer surface.
However, without relation to a layer forming method to be introduced, bond strength of the NiP layer with the glass substrate is rather poor, and it is essential to promote the strength. As a method for improving the bond strength, for example, it is possible to introduce an adhesive layer such as Cr, Ti under the NiP layer, and form the surface of the glass substrate with a surface roughness (mean value) Ra of about 0.5 xcexcm. However, the former method increases manufacturing processes and the latter method increases costs. Moreover, when the NiP layer is formed by the plating method, the thickness becomes about several xcexcm. The impact resistance of the medium depends to a large extent on the hardness of the NiP layer rather than on the hardness of the substrate. As a result, the impact resistance of the medium is lowered. Moreover, the NiP layer formation by the plating method only provides a smooth surface by conducting the polish process to the surface of the NiP layer.
Accordingly, it is desirable to provide a magnetic recording medium which assures good S/N equivalent to or better than that of an NiP layer, even when the NiP layer is not used.
Therefore, the first object of the present invention is to provide a magnetic recording medium which assures higher S/N.
The second object of the present invention is to provide a magnetic recording medium which assures higher impact resistance.
The third object of the present invention is to provide a magnetic recording medium with a seed layer having excellent contact property with a glass substrate.
The fourth object of the present invention is to provide a magnetic recording medium which is easier to manufacture.
A magnetic recording medium of the present invention has a first underlayer formed on a non-magnetic substrate to include material having any one of Co and Cu added to P. The medium also has a second underlayer consisting of Cr-based non-magnetic material between the first underlayer and the magnetic layer.
According to the present invention, the recording density of the magnetic recording medium can be improved because a S/N ratio similar to that of magnetic recording media having a NiP layer can be obtained by providing the first underlayer on the substrate. The S/N can be further improved by performing an oxidation process to the surface of the first underlayer. Moreover, the first basic layer of the present invention assures good adhesion property with glass, unlike the NiP layer, and does not require an adhesive layer to enhance adhesive property with the glass substrate. Therefore, manufacture can be simplified and manufacturing cost can also be reduced. In addition, since adhesive property of the first underlayer and glass substrate can be improved, use of the glass substrate is easier and higher impact resistance can be assured.